The present invention relates to a thermostable and corrosion-resistant cast nickel-chromium alloy.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
High-temperature processes, for example those used in the petrochemical industry, require materials which are not only heat-resistant but also sufficiently corrosion-resistant and in particular are able to withstand the loads imposed by hot product and combustion gases. For example, the tube coils used in cracking and reformer furnaces are externally exposed to strongly oxidizing combustion gases with a temperature of up to 1100° C. and above, whereas a strongly carburizing atmosphere at temperatures of up to 1100° C. prevails in the interior of cracking tubes, and a weakly carburizing, differently oxidizing atmosphere prevails in the interior of reformer tubes at temperatures of up to 900° C. and a high pressure. Moreover, contact with the hot combustion gases leads to nitriding of the tube material and to the formation of a layer of scale, which is associated with an increase in the external diameter of the tube by a few percent and a reduction in the wall thickness by up to 10%.
By contrast, the carburizing atmosphere inside the tube causes carbon to diffuse into the tube material, where, at temperatures of over 900° C., it leads to the formation of carbides, such as M23C6, and, with increasing carburization, to the formation of the carbon-rich carbide M7C3. The consequence of this is internal stresses resulting from the increase in volume associated with the carbide formation or transformation and a decrease in the strength and ductility of the tube material. Furthermore, graphite or dissociation carbon may form in the interior of the tube material, which can, in combination with internal stresses, lead to the formation of cracks, which in turn cause more carbon to diffuse into the tube material.
Consequently, high-temperature processes require materials with a high creep strength or limiting rupture stress, microstructural stability and resistance to carburization and oxidation. This requirement is—within limits—satisfied by alloys which, in addition to iron, contain 20 to 35% of nickel, 20 to 25% of chromium and, to improve the resistance to carburization, up to 15% of silicon, such as for example the nickel-chromium steel alloy 35Ni25Cr-1.5Si, which is suitable for centrifugally cast tubes and is still resistant to oxidation and carburization even at temperatures of 1100° C. The high nickel content reduces the diffusion rate and the solubility of the carbon and therefore increases the resistance to carburization.
On account of their chromium content, at relatively high temperatures and under oxidizing conditions the alloys form a covering layer of Cr2O3, which acts as a barrier layer preventing the penetration of oxygen and carbon into the tube material beneath it. However, at temperatures over 1050° C., the Cr2O3 becomes volatile, and consequently the protective action of the covering layer is rapidly lost.
Under cracking conditions, carbon deposits are inevitably also formed on the tube inner wall and/or on the Cr2O3 covering layer, and at temperatures of over 1050° C. in the presence of carbon and steam, the chromium oxide is converted into chromium carbide. To reduce the associated adverse effect on the resistance to carburization, the carbon deposits in the tube have to be burnt from time to time with the aid of a steam/air mixture, and the operating temperatures generally have to be kept below 1050° C.
The resistance to carburization and oxidation is further put at risk by the limited creep rupture strength and ductility of the conventional nickel-chromium alloys, which lead to the formation of creep cracks in the chromium oxide covering layer and to the penetration of carbon and oxygen into the tube material via the cracks. In particular in the event of a cyclical temperature loading, covering layer cracks may form and also the covering layer may become partially detached.
Tests have revealed that microstructural phase reactions, in particular at higher silicon contents, for example of over 2.5%, evidently lead to a loss of ductility and to a reduction in the short-time strength.
It would therefore be desirable and advantageous to inhibit the damage mechanism of carburization—reduction in the creep rupture strength or limiting rupture stress—internal oxidation, with the further result of increased carburization and oxidation, and to provide an improved casting alloy which still has a reasonable service life even under extremely high operating temperatures in a carburizing and/or oxidizing atmosphere.